1. Field of the Invention
This invention relates to an Albada type inverted Galileo finder, and particularly to an Albada type inverted Galileo finder which is small in size and free from a ghost image.
2. Description of the Prior Art
In a lens shutter camera, an Albada type inverted Galileo finder has been exclusively used as a finder. With the recent trend of miniaturization of cameras, not only a master lens for photography but also a finder system have been demanded to be the small type. Furthermore, the fact that aberrations are not only fully corrected but there is no ghost image is the object for appraisal.
At present, the size in the vertical direction to the optical axis of most of inverted Galileo finders are designed to be determined by the effective diameter of an objective lens, and therefore it is important to reduce the effective diameter of the objective lens. The effective diameter can be reduced if the finder magnification is reduced, but in this case, an object appears to be small and is hard to see.
On the other hand, as an example, Japanese Patent Application Laid-Open No. 91,618/1986 discloses the art in which an ocular lens is thickened to shorten an optical path thereby reducing the effective diameter of an objective lens. However, if this is formed from a plastic lens, the wall-thickness of the ocular lens is too large. Therefore, in the injection molding, there are disadvantages as follows:
(a) Since there is a difference in cooling time between a peripheral portion and a center portion, there produces a contraction in a direction of an optical axis and a surface precision becomes deteriorated. PA1 (b) To maintain the surface precision, control by precise (pressure - volume - time) is required; it takes much time for cooling; and cost increases. PA1 (1) (incident light).fwdarw.r.sub.4 (reflection).fwdarw.r.sub.3 (reflection).fwdarw.r.sub.6 PA1 (2) (incident light).fwdarw.r.sub.5 (reflection).fwdarw.r.sub.4 (reflection).fwdarw.r.sub.6 PA1 (3) r.sub.5 (optical frame).fwdarw.r.sub.3 (reflection).fwdarw.r.sub.6 l PA1 di: On-the-axis thickness of the ith lens from the object side in the group of ocular lenses PA1 m: number of lenses PA1 C.sub.l ': curvature of a reflecting surface of Albada system PA1 h.sub.1 : paraxial incident height of on-the-axis ray to the ith surface PA1 .alpha..sub.1 : paraxial angle of inclination of on-the-axis ray in an object space, and when a position of an object is infinite remote, .alpha..sub.1 =0 PA1 h.sub.i : paraxial incident height of principal ray at the ith surface PA1 .alpha..sub.1 : paraxial angle of inclination of principal ray at the object space PA1 e.sub.i ': Converted surface spacing from the ith surface to the next surface, that is, if the surface spacing from the ith surface to the next surface is t.sub.i ' and the refractive index is N.sub.i ', e.sub.1 '=t.sub.i '/N.sub.i ' PA1 .gamma.: finder magnification PA1 .phi..sub.A : refracting power of the second lens as the lbada system PA1 C.sub.1 : curvature of the surface of the second lens on the object side PA1 C.sub.2 : curvature of the surface of the second lens on the pupil side PA1 n: refractive index of the second lens
Furthermore, as an example, Japanese Utility Model Application Laid-Open No. 8,935/1985 discloses the art in which an intermediate optical member is interposed between an ocular lens and an objective lens to shorten an optical path. In this case, however, since a surface of a concave lens on the side of the pupil is made to comprise a reflecting surface of the Albada system, if the curvature of the reflecting surface is small, a ghost occurs. If the curvature is made to be increased in consideration of the ghost, the image surface of the Albada system greatly tends i a direction of negative diopter; the negative refracting power of an ocular lens group becomes great; and the negative distortion aberration of the finder system becomes great. Furthermore, the distance between the principal points of the objective lens group and the ocular lens group becomes small, and the positive refracting power of the ocular lens group need be strengthened and the comatic aberration of the Albada system remarkably occurs. If the magnification is increased to increase the field angle, the aforesaid various aberrations become further deteriorated.